The on-load tap switch changes the primary and secondary ratio of transformer windings by switching the multi-tap transformer from one tap to another when the transformer is loaded, thereby changing the output voltage of the transformer.
In the prior art, the combined on-load tap switch usually includes a switching means and a tap selector. Generally speaking, the switching means is arranged at the top, while the tap selector is arranged at the bottom, and the switching means and the tap selector are arranged co-axially and mechanically connected. The combined on-load tap switch is placed into the oil tank of the transformer from the flange hole positioned on the upper cover of the transformer.
The combined on-load tap switch usually takes on a column form and is sealed inside an independent column oil chamber, while the tap selector takes on a cage-shaped structure with a plurality of connecting terminals, which are used for connecting correspondingly with the coil tap of the transformer and submerged in the insulation medium contained in the oil tank of the transformer.
The above mentioned structure has following disadvantages:                (1) A space needs to be reserved in the transformer for the switch, thereby increasing the volume of the transformer;        (2) The failure maintenance and regular examination for the switch are relatively troublesome, as the transformer needs to be fully discharged with oil and covered with suspension, thus resulting in relatively a high maintenance cost and long maintenance time; and        (3) The transformer may be polluted from potential leakage or damage in the oil chamber containing the switch, thereby causing unforeseen losses and further problems.        
The prior external hanging on-load tap switch includes a combined on-load tap switch which only combines switching and selecting functions, that is, a transition contactor, an arc contactor, a transition resistor and a main contactor are combined in a movable contactor assembly, thereby achieving the operation of transitioning and switching through the rotation of the main shaft of the switch.
As shown in FIG. 1, a combined switch includes an auxiliary contactor 11, a main contactor 12, a transition contactor 13 and a transition resistor 15, which are combined and assembled on a movable contactor bracket 14 to form a movable contactor assembly. The movable contactor assembly achieves the combined operation of transition and switching at one time through the rotation of the insulated central shaft. Limited by its structure, the combined on-load tap switch is useful only for relatively low voltages and small capacity on-load tapping, and not suitable for use in high voltages and large currents.
Additionally, the combined on-load tap switch available in the market, such as the type M switch produced by the MR Company in Germany, is used for changing the transforming ratio while an oiled transformer is loaded. The type M switch includes a switching means and a tap selector to form a single-post structure. However, as the type M switch only has a Y-type connection mode, a large Δ connection transformer would need three type M switches, each of which respectively controls one phase of the transformer, thereby increasing the volume of the transformer, as well as wiring difficulty and cost.